The conversion of oxygenates to olefins (OTO) is currently the subject of intense research because it has the potential for replacing the long-standing steam cracking technology that is today the industry-standard for producing world scale quantities of ethylene and propylene. The very large volumes involved suggest that substantial economic incentives exist for alternate technologies that can deliver high throughputs of light olefins in a cost efficient manner. Whereas steam cracking relies on non-selective thermal reactions of naphtha range hydrocarbons at very high temperatures, OTO exploits catalytic and micro-architectural properties of acidic molecular sieves under milder temperature conditions to produce high yields of ethylene and propylene from methanol.
Current understanding of the OTO reactions suggests a complex sequence in which three major steps can be identified: (1) an induction period leading to the formation of an active carbon pool (alkyl-aromatics), (2) alkylation-dealkylation reactions of these active intermediates leading to products, and (3) a gradual build-up of condensed ring aromatics (coke or coke-like molecules). OTO is therefore an inherently transient chemical transformation in which the catalyst is in a continuous state of change. The ability of the catalyst to maintain high olefin yields for prolonged periods of time relies on a delicate balance between the relative rates at which the above processes take place. The formation of coke-like molecules is of singular importance because their accumulation interferes with the desired reaction sequence in a number of ways. In particular, coke renders the carbon pool inactive, lowers the rates of diffusion of reactants and products, increases the potential for undesired secondary reactions and limits catalyst life.
Over the last several decades, many catalytic materials have been identified as being useful for carrying out the OTO reactions. Crystalline molecular sieves are the preferred catalysts today because they simultaneously address the acidity and morphological requirements for the reactions. Particularly preferred materials are eight-membered ring aluminosilicates, such as those having the chabazite (CHA) framework type, as well as silicoaluminophosphates of the CHA structure, such as SAPO-34. These molecular sieves have cages that are sufficiently large to accommodate aromatic intermediates while still allowing the diffusional transport of reactants and products into and out of the crystals through regularly interconnected window apertures. By complementing such morphological characteristics with appropriate levels of acid strength and acid density, working catalysts are produced. Extensive research in this area indicates that silicoaluminophosphates are currently more effective OTO catalysts than aluminosilicates. In particular, the control of the silica to alumina molar ratio is a key requirement for the use of aluminosilicates in OTO reactions. Nevertheless, aluminosilicate zeolites continue to be explored for use in OTO and appear to have yet undiscovered potential.
Molecular sieves are classified by the Structure Commission of the International Zeolite Association according to the rules of the IUPAC Commission on Zeolite Nomenclature. According to this classification, framework-type zeolites and other crystalline microporous molecular sieves, for which a structure has been established, are assigned a three letter code and are described in the Atlas of Zeolite Framework Types, 5th edition, Elsevier, London, England (2001).
One known molecular sieve for which a structure has been established is the material designated as AEI, which is a molecular sieve having pores defined by two sets of generally perpendicular channels each having a cross-sectional dimension about 3.8 Angstrom. Molecular sieves of the AEI framework type do not exist in nature, but a number of aluminophosphates and silicoaluminophosphates having the AEI framework type have been synthesized, including SAPO-18, ALPO-18 and RUW-18. Moreover, because of their small pore size, AEI-type molecular sieves have been reported as suitable catalysts for a variety of important chemical processes, including the conversion of oxygenates to olefins. See, for example, U.S. Pat. No. 5,095,163, incorporated herein by reference.
U.S. Pat. No. 5,958,370, incorporated herein by reference, discloses an aluminosilicate zeolite designated SSZ-39 and having a silica to alumina molar ratio greater than 10, such as 10 to 100. SSZ-39 is produced by crystallizing an aqueous mixture comprising active sources of a trivalent element, such as aluminum, and a tetravalent element, such as silicon, in the presence of certain cyclic or polycyclic quaternary ammonium cations, such as N,N-dimethyl-2,6-dimethylpiperidinium cations, as templating agents. The synthesis can be conducted in the presence of SSZ-39 seed crystals, and there is no disclosure of the presence or absence of fluoride ions in the synthesis mixture.
The highest silica to alumina ratio exemplified for SSZ-39 in U.S. Pat. No. 5,958,370 is a ratio of 51. In column 5, lines 56 to 61, the patent teaches that SSZ-39 can be synthesized directly only as an aluminosilicate, although it suggests that the silica to alumina mole ratio can be increased, potentially to produce an essentially aluminum-free material, by use of standard acid leaching or chelating treatments. However, as is shown in the Comparative Example 13 below, attempts to dealuminize SSZ-39 by acid leaching or chelation have met with only limited success and have failed to produce materials having a silica to alumina ratio greater than 100.
In an article entitled “Guest/Host Relationships in the Synthesis of the Novel Cage-Based Zeolites SSZ-35, SSZ-36 and SSZ-39”, J. Am. Chem. Soc., 2000, 122, pages 263-273 Zones and certain of the co-inventors from U.S. Pat. No. 5,958,370 discuss the synthesis and structure of the molecular sieves, SSZ-35, SSZ-36 and SSZ-39. According to this article SSZ-39 is isomorphous with the AEI framework type aluminophosphate molecular sieve SAPO-18 and is a frequently observed product of high-alumina containing syntheses using cyclic and polycyclic quaternized amine structure directing agents. In particular, the article reports that, although SSZ-39 is produced at silica to alumina mole ratios of 30 with a variety of directing agents, including N,N-dimethyl-2,6-dimethylpiperidinium compounds, when the silica to alumina mole ratio is increased to 40 or higher, other crystalline phases, such as SSZ-35 and MFI and MTW framework type materials, are produced.
Chabazite-based zeolites have also been made, some even using preparations that do not contain fluoride, but these reported materials generally have relatively low Si/Al ratios. For instance, U.S. Pat. No. 4,544,538 discloses the synthesis details of SSZ-13, which was experimentally made only at Si/Al ratios as low as 15, with a fluoride-free preparation. Additionally, U.S. Pat. No. 6,709,644 and International Publication No. WO 03/1020641 A1 both disclose the synthesis details of SSZ-62, though, because of the relatively low Si/Al ratios, these disclosures tend to concentrate on aspects of crystal size so as to attain product having less than 0.5 micron particle size.
The present invention relates to methods for converting oxygenates to olefins using aluminosilicates having a CHA framework type, having an intermediate Si/Al ratio, and that are made using a substantially fluoride-free preparation.